There are a variety of source types for delivering seismic energy into the earth. Each type of seismic source provides seismic energy that has common characteristics that can be observed in the data record. However, it is generally desired that seismic sources used in a survey are operable to create distinguishable characteristics so that the data record may be inverted for source separation. The raw data typically includes far more information than can be analyzed by the most powerful computers, so the data is processed to draw out the pertinent features for computer analysis.
An early processing step is source separation of the data record to identify the signals originating from each source and separating those signals from one another. A following step is de-signaturing each separate signal into a single spike of energy. The single spike represents the cumulation of the reflected signal from a substructure. A routine seismic survey will create a tremendous number of spikes. Even with the simplifying of the data by de-signaturing into a single spike, the computer processing for a routine survey is still quite substantial.
These processes of source separation and de-signaturing data is done regardless of whether the source signal is a land survey or marine survey, whether the sources are vibrators which typically create sweeps of seismic energy or elastic wave generators that produce impulsive signals similar to explosives, or an airgun array such as used offshore or a marine vibrator which are used occasionally to produce a sweep of energy.
Source separation is aided by sources that have profoundly different characteristics. However, it is more common to use similar sources throughout a survey, where the distinguishing characteristics of individual sources are typically not highly distinctive. Therefore, an accurate understanding of the characteristics of the seismic signal from each source that was applied at each source point location can substantially aid in source separation. Higher precision of the characteristics for the seismic source signal inputted into the earth provides higher precision in source separation.
As an example of how energy is input into the ground, in the process of acquiring land seismic data, it is conventional to use a seismic vibrator to input seismic energy into the ground. Seismic energy is generally applied over time where the vibrators begin a sweep by vibrating initially at a low frequency and progressively increase the frequency such that an entire sweep of the frequency range is delivered within a definite time. Sweeps of four to eight seconds have been standard practice for years, but longer sweeps are becoming increasingly common with twenty-four second sweeps and forty-eight second sweeps also being used.
The costs for a seismic survey can be quite expensive and much effort has gone into improving the efficiency of seismic surveying. One advance is to operate several seismic vibrators at the same time all making a similar sweep, but at different phases with respect to one another. In other words, if the baseplate of one vibrator were to be going up while another is going down, the two vibrators would be about 180 degrees out of phase. Operating four vibes that are out of phase with respect to one another is known and commercially in use as the high fidelity vibroseis (“HFVS”) or ZenSeis® geophysical prospecting systems among others. Thus, commonly four (or some other number of) vibrators can be delivering seismic energy at one time and are each identifiable in the recordings from all of the seismic receivers. Typically, with four vibrators, at least four separate sweeps are performed where the phase relationship between the vibrators is changed between sweeps to enhance the distinctiveness and identifiably of each vibrator in the data record.
With distinctive and identifiable sweeps, the problem then becomes one of time and effort efficiency for separating out the individual shot records from the composite data set. For HFVS and ZenSeis® system, the approach is by inversion and separation of the data using a source signal. The common technique in the industry is to use the ground force estimate provided by the vibe controller as a proxy for the true source signal. The problem is that the true vibe source signal is not precisely known. This leads to imperfect separation and poor resultant separated data. The fidelity of the inverted and separated data is a function of the precision of the proxy source signal. The more accurate the proxy source signal, the signals will be more accurately separated by source and the output will be higher quality. Thus, the issue then becomes: what is the true source signal? or how can a more accurate proxy source signal be obtained?
The land example above demonstrates some of the reasons that the source signal needs to be accurately known for the best results in high production vibratory sourcing techniques. The same concerns apply to land impulsive sources and to marine cases with airgun arrays and with marine vibrators.